1. Field of the Invention
This invention relates to a heat sensitive recording member, and more particularly, to a heat sensitive recording member of improved storage stability.
2. Description of the Prior Art
There have been known various types of heat sensitive recording members such as, for example, a heat sensitive recording member whose coating layer becomes transparent to reveal the color of a background when heated, that capable of producing visible images by the chemical reaction of two compounds by heating, and that capable of producing visible images by the heat decomposition of one compound. Among them, the heat sensitive recording member utilizing the chemical reaction of two compounds shows an excellent heat sensitive color forming characteristic to give sharp recording images and therefore, is suitable for use in the recent thermal print systems in which only a short heating time of the order of a millisecond is employed.
With respect to the color forming components used in the two compound type heat sensitive recording member, there are known, for example, a combination of a metal salt of a higher fatty acid and a chelating agent as disclosed in Japanese Patent Publication No. 8787/1957 and a combination of a dye base such as crystal violet lactone and a phenolic acidic compound as disclosed in Japanese Patent Publication No. 14039/1970. These combinations of color forming components have various drawbacks such as unusual color formation of the coating liquid during production, color formation during storage of the recording member for a long time, fading of the color formed images and color formation caused by pressure.
As mentioned above, conventional heat sensitive recording members have disadvantageously low humidity resistance and low temperature resistance, and furthermore, for the purpose of lowering the load of a heating element of a thermal printer and enhancing the velocity of the recording response, there have been recently demanded heat sensitive recording members capable of color formation at a low temperature region with a small quantity of heat. However, there have not yet been such heat sensitive recording members free from the above mentioned various drawbacks and positively satisfying the new requisites.
The combination of color forming components as used in the present invention, that is, an iron salt of a higher fatty acid (component-A) and gallic acid (component-B), are known, for example, by Japanese Patent Publication No. 8787/1957, but the single coating solution obtained by mixing the components-A and -B disadvantageously causes unusual color formation during storage before coating due to undesirable early initiative of the reaction of these components.
In order to inhibit such unusual color formation of the coating solution during storing before applying, there has been used a chelating agent such as oxalic acid, tartaric acid, citric acid and the like, gallic acid ester, or a reaction product of gallic acid-hexamethylene tetramine, but the inhibition of unusual color formation is not satisfactory and, in addition, the storage stability of the resulting heat sensitive recording member is lowered.
In general, the reaction of an iron salt of a fatty acid with gallic acid or a gallic acid ester is as shown below according to an experimental result.
______________________________________ Storing stability of Storing stability of Component-B the coating solution the recording member ______________________________________ Gallic acid poor good Gallic acid good poor esters ______________________________________
It has been very difficult to improve both stabilities simultaneously.